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Jacks and Lifting Tackle                    205

mechanical advantage. There is a very large loss by friction
(some 70%), but this resistance is useful as serving to sustain the
weight when the chain is released by the hand. (Seep. 1047.)

Jacks are useful where overhead support cannot easily be
obtained. Fig 217 shews a simple Bottle Jack, the 'bottle7
serving as a fixed nut in which the screw rises when turned by a
tommy bar; and Fig. 219 represents a more powerful Jack 'with
worm gear. Here the screw is prevented from rotating by the
jaw dy and is, therefore, raised by the rotation of the worm wheel
A, which acts as a nut. In the example a handle of 14 ins.
radius turns, by means of a worm, a worm wheel of 16 teeth,
enclosing a screw of ij in. pitch; and a weight of 10 tons is
thereby lifted. The lower jaw d is for loads that are near the
ground, and the jack may be traversed, when in .position, by the
ratchet arm c, applied to the screw b at either end.

The Hydraulic Jack is both very useful and very interesting,
and is shewn at Fig. 218. It has an upper and a lower jaw to
suit various work, and both are part of the cylinder A. B is a
reservoir in which is placed oil, or water and glycerine. The handle
being moved upward on the fulcrum c, the pump plunger D is
thereby raised, and the liquid enters the pump through the
suction valve E; on the down stroke it is forced through the
delivery valve F, and exerts a pressure behind the ram G, thus
lifting the cylinder A. The valves are c non-return/ being loaded ,
by springs, and the ram is packed by a cup leather. It being
required to lower, the screw-down valve H is released, and the
liquid runs back to the reservoir. Screw j is for filling the latter,
and K is an air hole to assist the pump suction. The power
obtained depends both on the leverage and on the ratio of the
areas of plunger and ram, and may be calculated in the same way
as for the hydraulic press, which is discussed at/. 736.

There are a few other small tools of use to the Erector. The
D Cramp A, Fig. 219^, is for temporarily fastening two pieces
of work together; and the Key Drift B for releasing keys when
fitting wheels upon shafts. The file c is provided with a special
handle, usually made from a bent bolt, to enable a very large
surface to be filed; and the Square Drift at Fig. 219^ is really
a Fitter's tool, being used to clean out square holes too small to be